U.S. Pat. No. 4,016,140 describes a heat treatment in a temperature range between 300.degree. F. and 700.degree. F. prior to molding. This treatment provides a molecular weight increase necessary for good copolymer properties and more complete imidization of the polymers. In addition, this treatment prevents the formation of water vapor which would cause void formation later in the molding step.
U.S. Pat. No. 4,167,620 to Chen describes the heat treating of polyimide resins to improve their physical properties. The process is used to remove volatiles generated and contained within the polyamide-amides and polyamic acids that will cause voids, delaminations and warping of a structure made from this material when heated to near the heat deflection temperature. A circulating gas is used to help flush the volatiles from the solid. The treatment is likewise a post cure which promotes oxygen enhanced crosslinkings within the polymer to increase the heat deflection temperature about 30.degree. F.
Yoshimura et al Pat. Nos. 4,401,590 and 4,497,728 are both concerned with processes which alter the conductivity of polyimides.
This is accomplished by heating the material to a temperature between 400.degree. C. and 1,000.degree. C.
Pater Pat. No. 4,560,742 describes the standard post cure for a polyimide PMR-15 matrix composite. The heating does not take place in an inert atmosphere and the composite material is heated to a temperature as high as 600.degree. F.
Edelman Pat. No. 4,395,51is directed to graphite reinforced polyimide resin materials having improved structural properties. After lay-up, the composite material is heated to temperatures as high as 635.degree. F. for comparatively short periods of time and not in an inert atmosphere. The main objective is to reduce curing temperature from 600.degree. F. to a temperature of about 350.degree. F. to 450.degree. F.